Priority is claimed to Korean Patent Application No. 00-79184, filed Dec. 20, 2000, herein incorporated by reference.
1. Field of the Invention
The present invention relates to a semiconductor laser diode and a method of manufacturing the same, and more particularly, to a semiconductor laser diode including a ridge wave guide and a method of manufacturing the same.
2. Description of the Related Art
Semiconductor lasers are widely used in a communications field such as optical communications or in apparatuses such as compact disk players (CDPs) or digital versatile disc players (DVDPs), as means for transmitting data or writing and reading data.
As described above, semiconductor lasers are widely used for various reasons, such as their ability to maintain the laser characteristics within an appropriate restricted space, the fact that they can be miniaturized, and, above all, for the reason that they have a small value of critical current for laser oscillation.
Due to an increase in the industrial field of semiconductor laser use, the need for of semiconductor lasers is increasing, and also semiconductor lasers having a small critical current value are in demand.
Accordingly, semiconductor lasers which have a reduced critical current value have been developed or are being currently developed. An example of the above-described semiconductor lasers is shown in FIG. 1, which shows a conventional semiconductor laser diode including a ridge wave guide in order to reduce the value of critical current for laser oscillation.
Referring to FIG. 1, an n-GaN layer 12 is included on a sapphire substrate 10. The n-GaN layer 12 can be divided into first and second regions R1 and R2. An n-AlGaN/GaN layer 24, an n-GaN waveguide layer 26, an active layer (lnGaN layer) 28, a p-GaN waveguide layer 30 and a p-AlGaN/GaN layer 32 are sequentially formed on the first region R1. The refractive indices of the n-AlGaN/GaN layer 24 and the p-AlGaN/GaN layer 32 are lower than those of the n-GaN waveguide layer 26 and the p-GaN waveguide layer 30. Also, the refractive indices of the n-GaN waveguide layer 26 and the p-GaN waveguide layer 30 are lower than that of the active layer 28. The p-AlGaN/GaN layer 32 has a ridge shape, the upper middle portion of which protrudes. The sides of the protruding portion are perpendicular to the surface surrounding the protruding portion, and the top of the protruding portion is perpendicular to the sides and flat. The width of current flowing to the p-AlGaN/GaN layer 32 is restricted by the protruding portion. As a result, a resonance region for laser oscillation in the active layer 28 is defined. A p-GaN layer 34 is formed on the top of the protruding portion of the p-AlGaN/GaN layer 32. The exposed entire surface of the p-AlGaN/GaN layer 32 is covered with a protective layer 36. Both ends of the p-GaN layer 34 excluding the middle portion which is a current passage contact the protective layer 36. A p-type electrode 38 is formed on the protective layer 36 and contacts the exposed entire surface of the p-GaN layer 34.
The second region R2 of the n-GaN layer 12, which is lower than the first region R1, has an n-type electrode 40 formed thereon.
As described above, in a conventional laser diode, the resonance width is restricted by the ridge structure, so that the value of critical current for laser oscillation is reduced compared to the case of a non-ridge structure. However, as shown in FIG. 2, when the height of a ridge is made low by shallow etching for forming the ridge, the resistance of the p-AlGaN/GaN layer 32 and the resistance of the p-GaN layer 34 greatly increase, and current flowing via the p-GaN layer 34 spreads over the width of a ridge before it reaches the active layer 28. As a result, a resonance region Al (FIG. 2) widens, so that a critical current value for laser oscillation may increase. On the other hand, as shown in FIG. 3, when a ridge is high and the p-AlGaN/GaN layer 32 around the ridge is thin, due to deep etching for forming a ridge, spreading of current can be prevented, but the portion under the ridge is included in an optical wave guide during laser resonance. Thus, light loss may occur, and, accordingly, the value of critical current may increase.
To solve the above problem, an objective of the present invention is to provide a semiconductor laser diode including a ridge wave guide designed to increase a resonance width while maintaining the value of critical current and to prevent light loss which occurs due to the structure of a conventional ridge in which the portion under a ridge is included in an optical waveguide during resonance.
Another objective of the present invention is to provide a method of manufacturing the semiconductor laser diode.
To achieve the first objective, the present invention provides a semiconductor laser diode in which a ridge protruding perpendicularly to an active layer is formed in one of first and second material layers in which stimulated emission occurs, the first and second material layers being formed over and under the active layer, respectively, and having lower refractive indices than the active layer, the semiconductor laser diode contacting an electrode via the ridge, wherein the side of the ridge is made up of at least two portions having different gradients. A portion of the ridge that is close to the active layer becomes wider as it gets closer to the active layer. The ratio of a portion C2 of the ridge, the width of which increases in the direction of the active layer, to a portion C1, the width of which is constant, C2/C1, is no greater than 2/1, preferably 1/2 or 1/3. The first material layer in which stimulated emission occurs includes: a first compound semiconductor layer formed on a substrate; a first cladding layer formed on the first compound semiconductor layer; and a first waveguiding layer formed on the first cladding layer, the first waveguiding layer having a greater refractive index than the first cladding layer. The second material layer in which stimulated emission occurs includes: a second waveguiding layer formed on the active layer; a second cladding layer formed on the second waveguiding layer, the second cladding layer having a smaller refractive index than the second waveguiding layer and including the ridge; and a second compound semiconductor layer formed on the entire upper surface of the ridge. An electrode having a different polarity from the electrode formed on the ridge is formed on the first compound semiconductor layer. The substrate, which is a highly resistive sapphire substrate, can be formed of silicon carbon SiC.
To achieve the second objective, the present invention provides a method of manufacturing a semiconductor laser diode, in which a ridge protruding perpendicularly to an active layer is formed in one of first and second material layers in which laser is emitted, the first and second material layers being formed over and under the active layer, respectively, and having lower refractive indices than the active layer, and an electrode is connected to the ridge, wherein the ridge is formed so that its side has at least two portions having different gradients. A portion of the ridge that is close to the active layer becomes wider as it gets closer to the active layer. The ridge is formed so that the ratio of a portion C2 of the ridge, the width of which gradually increases in the direction of the active layer, to a portion C1, the width of which is constant, C2/C1, is no greater than 2/1. More preferably, the ridge is formed so that the ratio C2/C1 of the portion C2 of the ridge, the width of which gradually increases in the direction of the active layer, to the portion C1, the width of which is constant, is 1/2 or 1/3.
The ridge is formed by the steps of: forming a mask pattern used to form the ridge, on the first material layer; etching the second material layer until the second material layer has a predetermined thickness, using the mask pattern as an etch mask; and removing the mask pattern. Here, the second material layer is etched by slant-radiating an ion beam onto the surface of the second material layer at a predetermined angle. The slant radiation angle is 10 to 70xc2x0, preferably, 30xc2x0. A first compound semiconductor layer is formed on a substrate. A first cladding layer is formed on the first compound semiconductor layer. A first waveguiding layer is formed on the first cladding layer, the first waveguiding layer having a greater refractive index than the first cladding layer. The second material layer in which stimulated emission occurs is formed by the steps of: forming a second waveguiding layer on the active layer; forming a second cladding layer on the second waveguiding layer, the second cladding layer having a smaller refractive index than the second waveguiding layer and including the ridge; and forming a second compound semiconductor layer on the entire upper surface of the ridge. An electrode having a different polarity from the electrode formed on the ridge is formed on the first compound semiconductor layer so that it is isolated from the active layer. An electrode having a different polarity from the electrode formed on the ridge is formed on the bottom surface of the substrate.
In a semiconductor laser diode having a ridge wave guide according to the present invention, a ridge is made up of a portion whose width is uniform and a portion which is connected from the portion having a uniform width and whose width increases in the direction toward an active layer. Thus, an increase in a resonance width due to dispersion of current is prevented, and also light loss is prevented from occurring during resonance due to the structure of a convention ridge in which the portion under the ridge is included in an optical waveguide. Therefore, the value of critical current for laser oscillation can be further lowered.